One thing I failed to fully notice 1st time round: mirrors.

I use to see a lot of them in London studios (no not that, I'm not in Oasis). Remember them at Abbey Road 3 (long time ago) and there was a place I used to use, Red Bus, off Edgware road that was covered in them.

Acoustically not great, I imagine they are quite reflective

Yeah get some fabric covering worse bits. (Near mics)